1. Field of the Invention
This invention resides in the field of cassettes containing slab-shaped gels for use in electrophoresis.
2. Description of the Prior Art
Slab gel electrophoresis is widely used in clinical and research laboratories for the analysis of biological samples in view of the ability of the procedure to separate and analyze a multitude of samples simultaneously, and the ease with which the separated components of any given sample can be observed and identified by their locations in the gel. The slab-shaped gels that serve as the separation medium in these procedures are often supplied in pre-cast form in a cassette, in which the gel is retained between two flat, usually transparent plates. Pre-cast gels eliminate the risk of operator error in the casting of the gel, and in general the variations that arise when the gels are prepared on an individual basis by the user or at the site where the gels are to be used. The use of pre-cast gels also reduces the time and labor required in the preparation for and performance of an electrophoretic separation.
In use, the cassette is placed in contact with electrodes through buffer solutions that provide full fluid contact with the gel along opposing edges of the gel. While the buffer solutions and the cell that supplies the electrical connections to the gel through the solutions are not part of the cassette, certain cassettes are constructed with an integrated reservoir whose cavity borders an exposed edge of the gel, so that a buffer solution can be placed in the reservoir to contact the gel edge without contacting other edges of the gel. Typically, a single such reservoir is located along the upper edge of the cassette to provide electrical contact with the upper edge of the gel, while electrical contact with the lower edge of the gel is achieved by submerging the lower edge of the cassette in a pool of buffer solution in a tank. The upper and lower buffer solutions are thus fully separated, and electrodes submerged in the solutions supply the electrical potential that spans the gel uniformly along the width of the gel. A description and depiction of a cassette with an integrated upper buffer chamber of this type is found in Chu, D. Y., et al., U.S. Pat. No. 5,073,246, issued Dec. 17, 1991, and incorporated herein by reference. The integrated reservoir, in addition to accommodating the buffer solution, also accommodates the electrode that is immersed in the solution, and in many cases also provides access by which the “comb” or well-forming insert is placed along the upper edge of the gel space to form wells in the gel as the gel is being formed from the monomer solution.
Pre-cast gel cassettes are typically stored in refrigerated environments, and although they are most often oriented vertically during an electrophoretic procedure, they are typically stored horizontally in stacks in laboratory refrigerators. Regardless of whether the cassettes are laid horizontally or allowed to stand vertically, the integrated upper buffer chambers make stacking difficult and consume storage space which is typically at a premium in a laboratory refrigerator. The added volume consumed by an integrated chamber is evident from the figures and description in Chu et al. referenced above, and is likewise evident in certain commercially sold cassettes, notably Bio-Rad CRITERION® cassettes and precast gels. The present invention addresses this problem.